In automobile engines, in order to prevent tile escape of fuel vapor from a fuel tank into the atmosphere, the vapor is for example adsorbed by a canister of active carbon, and the adsorbed fuel is purged in the intake passage of the engine under predetermined running conditions. The fuel is removed from the canister and led into the intake passage by making use of the pressure difference between the atmosphere and the intake passage. For this purpose, the canister and intake passage are connected by a purge line, a purge valve which operates under predetermined running conditions being interposed in this line. Purge gas flowing into the intake passage from the purge line is led into the engine cylinder together with fuel injected by an injector provided in the intake passage, and burnt.
In many automobile engines which employ a three-way catalyst to process the engine exhaust gas, the air-fuel ratio (AFR) of the fuel mixture provided to the engine cylinder is feedback-controlled to stay in a certain region of a theoretically defined value. For example, a basic fuel amount is computed based on the intake volume of the engine, and this basic fuel injection amount is corrected based on the actual AFR detected by an oxygen sensor provided in an exhaust passage of the engine.
When the aforesaid purge gas is introduced, the fuel amount supplied to the engine cylinder increases by an amount corresponding to the purged fuel, and naturally the AFR becomes richer. In an engine with the above AFR feedback control system, the injection amount from the injector is corrected so that it is decreased.
If for example the detected AFR changes from lean to rich with reference to the theoretical value, an updating amount P is subtracted in one step from the feedback correction coefficient .alpha., and an updating I are then subtracted integrally until the AFR next changes back to lean. This is so-called "PI control". The correction coefficient .alpha. can therefore be varied only in fixed proportions, and if the AFR is varying rapidly, a certain response time is required until the AFR can be made to converge to a target value.
If now the accelerator pedal is depressed so that the vehicle accelerates while fuel is being purged from the canister, the intake amount immediately increases, but the flowrate of purge gas does not vary so much. The fuel supplied to the engine is therefore insufficient due to the fact that the purge gas amount has decreased relative to the intake amount, so this deficiency is compensated by feedback control which increases the fuel amount injected by the injector.
Just before acceleration, however, the feedback control correction coefficient .alpha. had shifted from a center value of 1.0 to lean (e.g. 0.8) so as to make the AFR converge to the target value. A relatively long time is therefore required for the coefficient to change back to rich which is necessary to increase the fuel amount so that the vehicle can accelerate, and during this time the engine does not respond properly.
To deal with this problem, in Tokkai Hei 2-19631 published by the Japanese Patent Office, a method is proposed whereby the basic fuel amount is first decreased by a predetermined value while purge gas is being led into the cylinder, the AFR is modified to the target value (theoretical AFR) by feedback control, and the correction coefficient .alpha. is maintained close to the center value of 1.0 even during purge. For this purpose, the AFR correction coefficient .alpha. before starting purge is compared with the AFR correction coefficient .alpha. after purge which has fallen to a stable level below the predetermined value. By first subtracting a fuel correction amount corresponding to this difference from the basic fuel amount, the correction coefficient .alpha. during purge is thereby forcibly maintained in the region of 1.0 which is the center value. When the accelerator pedal is depressed, therefore, the feedback control correction coefficient .alpha. shifts to rich from the region of 1.0. Compared to the case when it shifts to rich from lean, the fuel amount increases more rapidly, hence the engine acceleration response is improved.
However, after starting purge, the basic fuel amount is only corrected after the correction coefficient .alpha. has almost stabilized to a constant value, and until this occurs, the only correction applied is that of feedback control. As mentioned hereintofore, the feedback correction coefficient .alpha. can be varied only in fixed proportions. Therefore, it cannot immediately respond to rapid fluctuations of AFR due to purge, and AFR errors tend to be large for a certain period after starting purge. Further, if the accelerator pedal is depressed during this period, the engine response worsens as stated hereintofore.
A major factor which affects the precision of AFR control during purge is fluctuation of the amount of purge gas. The amount of fuel led into the intake passage during purge is supposed to be constant, however the fuel amount which is actually led into the intake passage largely varies due to the state of the fuel adsorbed by the canister at that time or the pressure difference before and after the purge valve. The flowrate of fuel led into the intake passage during purge is not always constant, and even if the fuel injection amount from the injector is corrected by learning control or feedback control, the correction conditions largely vary. When the concentration or flowrate of purged fuel is high, therefore, a long time is required to make the AFR converge to the target value, and the exhaust gas composition worsens due to AFR errors during this period.
In order to reduce the effect of AFR variation due to fluctuation of the purge fuel amount, the absolute flowrate of purge gas must be reduced, but if this is done the time required to purge all of the adsorbed fuel from the canister becomes longer. Moreover, even if the purge flowrate is very small, the precision of AFR feedback control is inevitably less compared to the case when purge is not carried out. It is therefore desirable to avoid long purge times, and to complete purge within a short time interval.